'Hopping' Fish Suggests Walking Originated Underwater

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Air-breathing fish that can hop and walk across the floor on
their fins hint that walking may have evolved underwater before
such animals began migrating on to land, scientists find.

The
distant ancestors of humans and all mammals, reptiles, birds,
amphibians and other four-limbed animals, or tetrapods, are fish
that eventually developed the ability to breathe on land. One of
the few living fish related to these ancient land-dwellers are
air-breathers known as lungfish, which are found today in Africa,
South America and Australia.

Now scientists find that an African lungfish (Protopterus
annectens) can lift its body clear off the floor and propel
itself forward using scrawny "limbs," abilities previously
thought to have originated in early tetrapods.

"This shows us — pardon the pun — the steps that are involved in
the
origin of walking," said researcher Neil Shubin at the
University of Chicago. "What we're seeing in lungfish is a very
nice example of how bottom-walking in fish living in water can
easily come about in a very tetrapod-like pattern."

The lungfish in question has an eel-like body and a pair of
flimsy hind fins.

"If you showed me the skeleton of this creature and asked me to
make a bet on whether it walks or not, I would have bet it
couldn't," Shubin said. "Their fins seem like the furthest thing
from walking appendages possible."

The
lungfish's history makes them popular pets among
paleontologists, and anecdotes and rumors circulated among
scientists for years about walking behavior allegedly seen in
these strange fish. To uncover the truth behind these stories,
the researchers designed a special fish tank in which they could
record a lungfish's motions on camera from the side and below for
in-depth analysis.

Their videos revealed the lungfish commonly used its hind or
pelvic limbs to "bound," moving both limbs at once like one would
hop, and to "walk," alternating limb motions. [ Video
of lungfish walking ]

"I find it exciting and surprising that even with such small
fins, this lungfish is able to not only propel itself, but lift
its body clear offthe bottom as well," researcher Heather King,
an evolutionary biologist at the University of Chicago, told
LiveScience. "If you were to look at just the skeleton of the
lungfish, you might never guess that it was capable of this
behavior, especially since they don't have feet!" she wrote in an
email.

The forelimbs look similar to the hind limbs, but were not
involved in such bounding and walking movements. The researchers
are not certain why that is, but "it is possible that if the hind
limbs alone are sufficient for propelling the fish, the forelimbs
would not be used," King said.

The lungfish's ability to support its body on its slim limbs
might be aided by their buoyant air-filled lungs, researchers
suggest.

This discovery might redraw the evolutionary route scientists
think life took from water to land. Many of the steps needed to
adapt to surface-dwelling could have occurred millions of years
before early tetrapods developed limbs with digits and took their
first steps on shore, King said.